When preparing water-based inks for writing instruments, particularly when preparing inks falling in a low viscosity area (50 mPa·s or less: measured (25° C.) by means of an ELD type viscometer manufactured by Tokimec Inc.), a kind and an addition amount of material having a large penetrating rate into paper such as a surfactant having a low molecular weight have so far been restricted because a non-feathering property can not be expected.
In particular, in the case of a specification in which a metal lubricity between a ball and a tip holder is required as is the case with a ballpoint pen, a surfactant has to be indispensably added. However, addition of a surfactant of an amount which is enough for satisfying the metal lubricity results in deteriorating the non-feathering property.
Also, in the case of a structure requiring no metal lubricity as is the case with a marking pen, there is the problem that the drawn lines are blurred by an effect brought about by an emulsifier contained in a colored emulsion which is used as a colorant.
Further, it is possible to improve a non-feathering property by making an ink viscosity to be a non-Newtonian viscosity. Particularly in the case of a pigment ink, the drawn lines have a good quality, but color separation is caused by settling of the pigment with the passage of time at high temperature depending on the kind of a non-Newtonian viscosity-providing agent; while no problems are involved in settling of the pigment with the passage of time at high temperature, blobbing and splitting in the drawn lines are brought about, and the drawn lines are reduced in a quality; and the writing feeling is not improved even by adding a low molecular surfactant. Thus, there exists the problem that all qualities can not be satisfied at the same time.
On the other hand, known is a pigment ink for a ballpoint pen which comprises at least a pigment, water, a water-soluble organic solvent and a water-soluble polymer as a viscosity-controlling agent and in which an ink viscosity falls in a range of 50 to 2000 mPa·s (25° C.), wherein a specific polyglycerin fatty acid ester such as hexaglycerin monolaurate is added thereto, whereby the pigment ink is provided together with the characteristics of an oil-based ink and a water-based ink for a ballpoint pen, and it is less reduced in pigment dispersion stability with the passage of time (for example, Japanese Patent Application Laid-Open No. 38474/1990).
However, in the above pigment ink for a ballpoint pen described in Japanese Patent Application Laid-Open No. 38474/1990, the specific polyglycerin fatty acid ester such as hexaglycerin monolaurate functions as a pigment dispersant, and the ink is still inferior in terms of writing feeling, a non-feathering property and a drawn line-drying property.
Further, known is a water-based pigment ink composition for a ballpoint pen characterized by comprising at least a kind of alkylene oxide adduct of polyhydric alcohol selected from the group consisting of alkylene oxide adducts of polyglycerin, alkylene oxide adducts of glycerin in which 1 to 150 moles of alkylene oxide are added to glycerin, alkylene oxide adducts of trimethylolpropane and mixtures thereof, a pigment, a dispersant and water, and the ink composition makes the written lines less liable to cause feathering, has a satisfactory lubricity and provides good writing feeling (for example, Japanese Patent Application Laid-Open No. 186730/1993). In the above water-based pigment ink composition for a ballpoint pen described in Japanese Patent Application Laid-Open No. 186730/1993, the foregoing alkylene oxide adduct of polyhydric alcohol to which a fatty acid as a hydrophobic group is not bonded has a purpose of improving lubricity, and therefore the ink composition is still inferior in terms of writing feeling, a non-feathering property and a drawn line-drying property.